I hope you are in good health and iman. A friend of mine has a lot of waswas, shaytan tells her to whisper that she is a disbeliever. She then regrets and tries to ask for tawbah (forgiveness). What shall she do?
Firstly, she needs to strive to stop this waswas. Whenever this happens, she should make dhikr (remembrance of Allah) and say the Shahadah (testimony). She must fight back the was was.
Tawbah is enough to erase any sin, even kuffr (disbelief). She must regret and make istighfar.